By adaptation of a safe and noninvasive technique used in adults, we are measuring lung diffusing capacity, effective pulmonary blood flow, lung tissue volume, oxygen consumption, total lung volume and functional residual capacity in infants to prove the hypothesis that reduced oxygen or carbon monoxide diffusing capacity and altered blood flow in the lungs caused by an increase in lung tissue volume may worsen the respiratory distress syndrome in infants. In addition, it is hoped that by examining the data obtained from these critically ill infants, we can separate heart from lung disease so thad rapid therapeutic modalities to relieve either of these problems can be started without delay. The technique can also be used to assess the results of therapy so that by repeating the rebreathing study immediately after changing the ventilatory mode or oxygen concentration, one can assess the direct benefits of the change on the infant's cardiovascular and pulmonary systems. The adaptation of the adult method requires the design of a low dead space, low resistance system that will not alter respiratory assistance the infant may already be receiving. The equipment required for this project includes a mass spectrometer and an on-line computer system for data processing.